certctl(8): Pacify formal POSIX sh syntax.

According to POSIX 2018, the syntax between `then' and `elif' and
`fi' must be a _non-empty_ list of commands:

compound_list : linebreak term

certctl(8): Pacify formal POSIX sh syntax.

According to POSIX 2018, the syntax between `then' and `elif' and
`fi' must be a _non-empty_ list of commands:

compound_list : linebreak term
| linebreak term separator
;
...
if_clause : If compound_list Then compound_list else_part Fi
| If compound_list Then compound_list Fi
;
else_part : Elif compound_list Then compound_list
| Elif compound_list Then compound_list else_part
| Else compound_list
;

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_10_02

NetBSD's sh(1) currently doesn't enforce this and allows an empty
sequence of commands, but let's not rely on that nonstandard quirk.

Noted in PR 57997.

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certctl(8): Avoid basename(1).

Saves some time running subprocesses. Since this is only used for
non-directories (i.e., there's never trailing / on the inputs), it
suffices to delete the longest pr

certctl(8): Avoid basename(1).

Saves some time running subprocesses. Since this is only used for
non-directories (i.e., there's never trailing / on the inputs), it
suffices to delete the longest prefix matching glob `*/' with shell
parameter expansion -- much cheaper than spawning a subprocess.

Shaves off about 1/3 of the time spent in `certctl list' on an
aarch64 VM in qemu.

PR bin/57993

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