[RFC] Fix two android compile errors in fs.c

* Replace usage of deprecated S_IEXEC with S_IXUSR:

As noted in http://www.gnu.org/savannah-checkouts/gnu/libc/manual/html_node/Permission-Bits.html, "S_IEXEC is an obsolete synonym provided for BSD compatibility", so should be a general improvement to avoid using outdated constructs.

* Rename a variable named "errno" to avoid clashing with define:

As noted in http://man7.org/linux/man-pages/man3/errno.3.html, "errno may be a macro", so having a variable  named errno should be avoided in general.