ash: fix breakage of ${v/pat/str}

ash: fix breakage of ${v/pat/str}

The commit

   ash: move parse-time quote flag detection to run-time

breaks pattern substitution in parameter expansion.  Fix this and
revise the code so that the different handling of the pattern and
the replacement string takes place in rmescapes rather than the
separate function parse_sub_pattern.

function                                             old     new   delta
rmescapes                                            227     273     +46
static.qchars                                          3       4      +1
subevalvar                                          1177    1157     -20
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(add/remove: 0/0 grow/shrink: 2/1 up/down: 47/-20)             Total: 27 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>