Clean up execl*(3) manual page prototype formatting

Rendering of execle was missing a comma between the NULL argument and envp.

For unclear reasons, POSIX' definition of these routines comments out the
mandatory trailing NULL argument. That seems unnecessary and probably
(reasonably) confuses mdoc.

For unclear reasons, POSIX' definition of these routines spells NULL as
"(char *)0." This is needlessly unclear. One guess might be that POSIX
targets more exotic computer architectures than FreeBSD does. Fortunately,
there is no such problem on any reasonable platform for FreeBSD to support.
Spell NULL as NULL.

The comma was probably removed in r117204 while the comment and creative
spelling of NULL were added in r116537 (both 15 years ago).