Fix type of startup_with_shell in gdbserver

Fix type of startup_with_shell in gdbserver

startup_with_shell was changed to be of "bool" type, but I noticed
that the definition in gdbserver disagreed.  This disagreement caused
some regressions on a big-endian machine.

This patch removes the redundant declaration and definition of
startup_with_shell and ensures that such clashes will be diagnosed.

This moves the declaration to common-inferior.h, and introduces a new
common-inferior.c, as suggested by Pedro.

gdb/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* Makefile.in (COMMON_SFILES): Add common-inferior.c.
	* gdbsupport/common-inferior.c: New file.
	* infcmd.c (startup_with_shell): Don't define.
	* nat/fork-inferior.h (startup_with_shell): Don't declare.
	* gdbsupport/common-inferior.h (startup_with_shell): Declare.
	* inferior.h (startup_with_shell): Don't declare.

gdb/gdbserver/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* Makefile.in (SFILES): Add common-inferior.c.
	(OBS): Add common-inferior.o.
	* server.c (startup_with_shell): Don't define.