BUG/MINOR: log: missing some ARGC_* entries in fmt_directives()

BUG/MINOR: log: missing some ARGC_* entries in fmt_directives()

ARGC_CAP was not added to fmt_directives() which is used to format
error messages when failing to parse log format expressions. The
whole switch/case has been reorganized to match the declaration
order making it easier to spot missing values. The default is not
the "log" directive anymore but "undefined" asking to report the
bug.

Backport to 1.5 is not strictly needed but is desirable at least
for code sanity.