p4fastimport: fix ignored executable bits
Previous code looked for "text+x" type from 'p4 fstat' output. This was
insufficient, because files may have types like "text+Fx" or "text+wx".
Based on the information at
the executable bit is now properly handled in all cases.
The unnecessary regexp matching for the +k (keyword) file attribute is now
removed as well.
Differential Revision: https://phab.mercurial-scm.org/D1912