New test for follow-exec-mode

New test for follow-exec-mode

This patch implements a new GDB test for follow-exec-mode.  Although
there is a GDB test for debugging across an exec, there is no test for
follow-exec-mode.  This test is derived from gdb.base/foll-exec.exp,
and re-uses execd-prog.c as the program to exec.

The following behavior is tested:

follow-exec-mode == "same"
 - 'next' over the exec, check for one inferior
 - 'continue' past the exec to a breakpoint, check for one inferior
 - after the exec, use a 'run' command to run the current binary
follow-exec-mode == "new"
 - 'next' over the exec, check for two inferiors
 - 'continue' past the exec to a breakpoint, check for two inferiors
 - after the exec, use a 'run' command to run the current binary
 - after the exec, use the 'inferior' command to switch inferiors,
   then use a 'run' command to run the current binary

Note that single-step breakpoints do not survive across an exec.
There has to be a breakpoint in the execed program in order for
it to stop right after the exec.

gdb/testsuite/ChangeLog:

	* gdb.base/foll-exec-2.c: New test program.
	* gdb.base/foll-exec-2.exp: New test.