Various procfs.c cleanups

Various procfs.c cleanups

While reading through procfs.c, I noticed a couple of cleanup
opportunities:

* Some comments and code allowed for portability across different
  targets.  Since procfs.c is Solaris-only for some time now, those can
  go.

* Likewise, there were some references to the old ioctl-based /proc left.

* The code still allowed for SYS_exec.  However, it is no longer present
  in either Solaris 11.3, 11.4, or Illumos.  Checking the OpenSolaris
  sources, I found that it had already been removed in 2010 well before
  the Solaris 11 release.

* Some blocks of #if 0 code can go:

** References to struct procinfo.{g,fp}regs_dirty which are no longer
   defined.

** Code handling the PR_ASLWP flag where <sys/procfs.h> has

#define	PR_ASLWP   0x00000040	/* obsolete flag; never set */

Tested on amd64-pc-solaris2.11.

	* procfs.c: Cleanup many comments.

	(READ_WATCHFLAG, WRITE_WATCHFLAG, EXEC_WATCHFLAG)
	(AFTER_WATCHFLAG): Replace by value.

	(MAIN_PROC_NAME_FORMAT): Inline ...
	(create_procinfo): ... here.

	(procfs_debug_inferior): Remove SYS_exec handling.
	(syscall_is_exec): Likewise.
	(procfs_set_exec_trap): Likewise.

	(syscall_is_lwp_exit): Inline in callers.
	(syscall_is_exit): Likewise.
	(syscall_is_exec): Likewise.
	(syscall_is_lwp_create): Likewise.

	(invalidate_cache): Remove #if 0 code.

	(make_signal_thread_runnable):  Remove.
	(procfs_target::resume): Remove #if 0 code.