Add a 'starti' command.

Add a 'starti' command.

This works like 'start' but it stops at the first instruction rather
than the first line in main().  This is useful if one wants to single
step through runtime linker startup.

While here, introduce a RUN_ARGS_HELP macro for shared help text
between run, start, and starti.  This includes expanding the help for
start and starti to include details from run's help text.

gdb/ChangeLog:

	* NEWS (Changes since GDB 8.0): Add starti.
	* infcmd.c (enum run_break): New.
	(run_command_1): Queue pending event for RUN_STOP_AT_FIRST_INSN
	case.
	(run_command): Use enum run_how.
	(start_command): Likewise.
	(starti_command): New function.
	(RUN_ARGS_HELP): New macro.
	(_initialize_infcmd): Use RUN_ARGS_HELP for run and start
	commands.  Add starti command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Starting your Program): Add description of
	starti command.  Mention starti command as an alternative for
	debugging the elaboration phase.

gdb/testsuite/ChangeLog:

	* gdb.base/starti.c: New file.
	* gdb.base/starti.exp: New file.
	* lib/gdb.exp (gdb_starti_cmd): New procedure.