Document when execl and execp entered unix.

For this, things are complicated. The first mention in the manual was in the 4th
edition manual (as an add on to exec(II)).  The 2nd and 3rd editions didn't have
these in the manual (either as a separate page, or as an add-on to exec(II)). We
don't have good 1st, 2nd or 3rd edition distributions to look in. However,
there's a tape labeled 'last1120c' that we do have. This tape contains the last
version of the V2 edition of the C compiler on it (just after C got struct). On
this tape there was a libc.sa archive that contains source for execl and
execp. This source is sufficiently different from the V5 sources (which are the
next ones we have sources for) and have a slightly different calling convention
than later sources, suggesting that the early date for the last1120c tape is
correct (in that era, the epoch changed every year, leading to a one or two year
ambiguity on when the files could have been modified) and it should be though of
as V2. Since this was also a time of compiler development, and the calling
convetions are known to be under evolution, and since the rest of the sources in
libc.sa are consistent, that's further evidence that V2 is likely. Finally, 2nd
edition was the last version to fully support the 11/20 because it lacked many
basic features and bell labs moved off it to the 11/45 as soon as they could
afford to buy one, around this time era. The unix manuals make it sound like V3
might have supported the 11/20, but the same intro could also be read to mean it
didn't, at all, and that V3 was the first rewrite for the 11/45 ahead of the
rewrite in C that came with V4.

Taken together, the evidence leans most heavily to V2 (90% IMHO), and slightly
to V3 (8%) or possibly V4 (2%). I've not put all this in the man page, but have
left it here in case someone notices in the future that V4 is the first manual
page for it.

lib/libc/gen/exec.3

@@ -316,6 +316,12 @@ The
 function appeared in
 .At v1 .
 The
+.Fn execl
+and
+.Fn execv
+functions appeared in
+.At v2 .
+The
 .Fn execvP
 function first appeared in
 .Fx 5.2 .