Add CurrentSecondsSinceEpoch for obtaining the current date and time

[fingolfin]

GAP cannot tell the time. NanosecondsSinceEpoch looks like the answer but
counts from an unspecified point — CLOCK_MONOTONIC, or mach_absolute_time
on macOS. Here NanosecondsSinceEpoch()/10^9 is 73750, the uptime.

23 packages in the distribution handle dates; yags, QGNAG and the
libsemigroups benchmarks shell out to date.

CurrentSecondsSinceEpoch() returns seconds since 1970-01-01 00:00:00 UTC, so
it feeds the calendar functions that already exist:

gap> DMYhmsSeconds( CurrentSecondsSinceEpoch() );
[ 11, 8, 2026, 23, 14, 29 ]

The epoch is guaranteed rather than assumed: POSIX pins CLOCK_REALTIME and
gettimeofday to 1970, and the time() fallback converts via gmtime,
checked against a C reference for every 86399-second step from 1970 to 2030.

The manual entry for NanosecondsSinceEpoch now says it does not count from
1970 and points here.

Naming. CurrentSecondsSinceEpoch now sits beside NanosecondsSinceEpoch,
which is not since the epoch. CurrentUnixTime and CurrentTime were the
alternatives; renaming is four lines.

make check: 0 failures in 314 files.

Written with Claude Opus 5 via Claude Code; reviewed by me. The commit lists
the tool as co-author.

[fingolfin]

I've not yet reviewed this AI generated PR myself. So you may wish to hold off a review until I did so to avoid potentially being exposed to slop.

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.05%. Comparing base (104ccfa) to head (cff6f87).

Files with missing lines	Patch %	Lines
src/gaptime.c	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6504      +/-   ##
==========================================
- Coverage   79.06%   79.05%   -0.02%     
==========================================
  Files         685      685              
  Lines      293835   293836       +1     
  Branches     8666     8665       -1     
==========================================
- Hits       232325   232296      -29     
- Misses      59710    59735      +25     
- Partials     1800     1805       +5     

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