Allow -DNO_STRICT_ERE to restore historic regex behavior

[sjg]

Summary

Allow restoring the behavior of '{' as described in regex(3).
Ie. only treat it as start of bounds if followed by a digit.

If NO_STRICT_ERE is not defined, the behavior introduced by
commit a4a801688c909ef39cbcbc3488bc4fdbabd69d66 is retained,
otherwise the previous behavior is restored.

[kevans]

lib/libc/regex/regcomp.c

910

It occurs to me that this is the BRE path, but it'd be good for the knob to be consistent across BREs and EREs- maybe just rename the knob to NO_STRICT_REGEX? I don't think I have any complaints otherwise.

[sjg]

Rename knob to NO_STRICT_REGEX