Add usage for commands in printcmd.c

Add usage for commands in printcmd.c

I noticed that the help for "info addr" did not include a "usage"
line; and when adding it I went through and fixed a few minor issues
in printcmd.c:

* Added usage lines to all commands
* Updated the help text for some commands
* Changed some help to use upper case metasyntactic variables
* Removed some dead code

Regression tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-03-29  Tom Tromey  <tromey@adacore.com>

	* printcmd.c (_initialize_printcmd): Add usage lines.  Update some
	help text.  Remove dead code.

gdb/testsuite/ChangeLog
2019-03-29  Tom Tromey  <tromey@adacore.com>

	* gdb.base/help.exp: Tighten apropos regexp.