Ensure help2man is writable

When not running as root, the shebang workaround requires write
permissions to modify help2man. Since the original file is read-only,
permission must be granted explicitly.

Also fix the double '/' in the /usr prefix while at it.

sysa/help2man-1.36.4/help2man-1.36.4.sh

@@ -17,8 +17,9 @@ src_compile() {
     make MAKEINFO=true
 
     # fix a broken shebang
+    chmod +w help2man
     tail -n +6 help2man > help2man.tmp
-    echo "#!/${PREFIX}/bin/perl" > help2man
+    echo "#!${PREFIX}/bin/perl" > help2man
     cat help2man.tmp >> help2man
     rm help2man.tmp
 }