Implement /proc/self/exe readlink[at] fallback in POST handler

Implement /proc/self/exe readlink[at] fallback in POST handler

Calling the readlink[at] syscall directly from the PRE handler defeats
the FUSE_COMPATIBLE_MAY_BLOCK (SfMayBlock) flag. Add a POST handler
that only explicitly calls the readlink[at] handler for the
/proc/self/exe fallback (this should be fine unless /proc is also
implemented as fuse in this process).

Adjust readlink[at] GENX_ and LINX_ syswrap macros to GENXY and LINXY.

https://bugs.kde.org/show_bug.cgi?id=493507