profile/systemd-osc-context: also escape $USER and $HOSTNAME

profile/systemd-osc-context: also escape $USER and $HOSTNAME

Claude pointed out that while rather unlikely, it is possible for the USER and
HOSTNAME environment variables to contain problematic characters that require
escaping. Now that that escaping no longer involves the fork+exec overhead of
calling sed, let's ensure those fields do get put through the escape routine.

Refactor __systemd_osc_context_escape so that it not only takes the value and
prints that, but also the format specifier used for emitting the field. That
way, we avoid an extra subshell (i.e. fork+pipe and all that overhead), and
combined with the previous commit, we now only spawn one subshell instead of
two per start= sequence.

That could be reduced to zero, if the "callers" were rewritten to something
like

    printf "\033]3008;start=%.64s;type=shell" "$systemd_osc_context_shell_id"
    __systemd_osc_context_common
    printf "\033\\"

but that will mean that the sequence is not emitted with a single write()
system call [which isn't really guaranteed currently either, just very likely],
so some background process could end up writing to the middle of the sequence,
thus losing that output and mangling the OSC3008 info. This in turn could be
overcome by building the whole sequence in a shell variable using 'printf -v'
and only printing at the end, but that would be a somewhat invasive change.