Collisions with private symbols

This causes issues with static libraries. If the library user defines a symbol with the same name as one of our internal symbols, they'll clash.

Solutions include:

* Use an external program to strip symbols from the lib
* Prefix internal symbols with `_mrsh_`: technically symbols beginning with `_` are reserved, but it's very unlikely to cause any issue in practice
* Prefix internal symbols with `mrsh__`: this requires un-marking some exported symbols, which Darwin doesn't support
* Suffix internal symbols with `_mrsh`: doesn't sound very idiomatic in C
* Prefix internal symbols with `mrshp_` or `mrshpriv_`

Related: make `mrsh_getopt` private

Also see https://github.com/swaywm/wlroots/issues/1892