fs: add two new modes "as" and "as+"

Two new modes 'as' and 'as+' are added, for opening files:

• 'as': append in synchronous mode,
• 'as+': read write in synchronous mode.

Fixes #18635 (closed) .

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

fs