[RFA] patch to add 'maint profile-gdb' command

[Jason Molenda]

This is a refresh of Tom Tromey's gdb profiling patch, originally here:
	http://sources.redhat.com/ml/gdb-patches/2000-q1/msg00022.html

Instead of profiling all of gdb, Tom's patch lets you profile a
specific command (or commands).  You enable profiling with the
'maint profile-gdb on' before the command(s) of interest, and
'maint profile-gdb off' (or exit) when you're finished.

configure and config.in both need to be regenerated after applying
this patch.  Your build must be compiled with --enable-profiling
for this feature to be enabled.

My only comments on this patch are (1) the documentation entry
could note that your gmon.out file will be overwritten each time
gdb is started, even if you don't do a profile-gdb on command[1], and
(2) the configure.in check for $enable_profiling could be embedded
in the AC_ARG_ENABLE() autoconf call.  It doesn't make any practical
difference, but it looks like tradition in gdb's configure.in is
to include this code inside the AC_ARG_ENABLE call.

 [1]  A bit of profiling happens before it can be turned
      off in captured_main().  This initial profiling will overwrite
      away any existing gmon.out.  At least it does with the gprof
      on Linux and FreeBSD systems.

No testsuite regressions are added with this patch.

This patch does not require approval for the 5.1 branch - it is
not something end users have cause to enable.

This patch does add a couple of ifdefs in main.c, aint.c to guard
the code, but this is necessary.  Obviously you can't compile in
profiling all the time (performance, portability), and you can't
make calls to the profiling system calls if you aren't compiling
-pg.

Jason