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.