gdb/testsuite: Explicitly return from main

gdb/testsuite: Explicitly return from main

I've been playing with a board file that forces every testcase to
include a header file that does something like:

  #define main __gdb_testcase_main

and then links an actual main() function that does some
initialization and then jumps to __gdb_testcase_main.

That runs into a number of testcases relying on main not having an
explicit return statement, like e.g.,:

 gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/catch-follow-exec.c:27:1: warning: non-void function does not return a value [-Wreturn-type]
 gdb/build/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/catch-signal.c:47:1: warning: non-void function does not return a value [-Wreturn-type]

We don't get those warnings without my board because it is valid to
not explicitly return from main.  There's an implicit "return 0;".

Since it doesn't hurt to be explicit, I've went ahead and added the
explicit return statements.

Also, a couple testcases either don't explicitly specify main's return
type, or return void.  Those are tweaked to explicitly return int.

gdb/testsuite/ChangeLog:

	* gdb.base/catch-follow-exec.c (main): Add explicit return
	statement.
	* gdb.base/catch-signal.c (main): Likewise.
	* gdb.base/condbreak-call-false.c (main): Likewise.
	* gdb.base/consecutive.c (main): Add explicit return
	statement and return type.
	* gdb.base/cursal.c (main): Add explicit return statement.
	* gdb.base/cvexpr.c (main): Likewise.
	* gdb.base/display.c (main): Add explicit return statement and
	return type.
	* gdb.base/dprintf-detach.c (main): Add explicit return statement.
	* gdb.base/endianity.c (main): Likewise.
	* gdb.base/execd-prog.c (main): Likewise.
	* gdb.base/gdb1090.c (main): Likewise.
	* gdb.base/info_qt.c (main): Likewise.
	* gdb.base/lineinc.c (main): Likewise.
	* gdb.base/load-command.c (main): Likewise.
	* gdb.base/macscp1.c (main): Likewise.
	* gdb.base/pr10179-a.c (main): Likewise.
	* gdb.base/quit-live.c (main): Likewise.
	* gdb.base/scope0.c (main): Likewise.
	* gdb.base/settings.c (main): Likewise.
	* gdb.base/stack-checking.c (main): Return int.
	* gdb.base/varargs.c (main): Add explicit return statement.
	* gdb.cp/ambiguous.cc (main): Likewise.
	* gdb.cp/anon-struct.cc (main): Likewise.
	* gdb.cp/anon-union.cc (main): Likewise.
	* gdb.cp/bool.cc (main): Likewise.
	* gdb.cp/bs15503.cc (main): Likewise.
	* gdb.cp/cplusfuncs.cc (main): Likewise.
	* gdb.cp/cttiadd.cc (main): Likewise.
	* gdb.cp/extern-c.cc (main): Likewise.
	* gdb.cp/filename.cc (main): Likewise.
	* gdb.cp/formatted-ref.cc (main): Likewise.
	* gdb.cp/mb-ctor.cc (main): Likewise.
	* gdb.cp/member-ptr.cc (main): Likewise.
	* gdb.cp/minsym-fallback-main.cc (main): Likewise.
	* gdb.cp/overload-const.cc (main): Likewise.
	* gdb.cp/paren-type.cc (main): Likewise.
	* gdb.cp/parse-lang.cc (main): Likewise.
	* gdb.cp/pr-1023.cc (main): Likewise.
	* gdb.cp/psmang1.cc (main): Likewise.
	* gdb.cp/readnow-language.cc (main): Likewise.
	* gdb.cp/ref-params.cc (main): Likewise.
	* gdb.cp/rvalue-ref-params.cc (main): Likewise.
	* gdb.cp/virtbase2.cc (main): Likewise.
	* gdb.dwarf2/dw2-abs-hi-pc.c (main): Likewise.
	* gdb.dwarf2/dw2-namespaceless-anonymous.c (main): Likewise.
	* gdb.dwarf2/dw4-toplevel-types.cc (main): Likewise.
	* gdb.mi/mi-console.c (main): Likewise.
	* gdb.mi/mi-read-memory.c (main): Likewise.
	* gdb.modula2/multidim.c (main): Likewise.
	* gdb.opt/inline-small-func.c (main): Likewise.
	* gdb.python/py-rbreak.c (main): Likewise.
	* gdb.stabs/exclfwd1.c (main): Likewise.
	* gdb.trace/qtro.c (main): Likewise.