factor out IS_SLASH, perdir fix

[covener]

in per-dir, the filename will be internally redirected, so / is OK too.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918651 13f79535-47bb-0310-9956-ffa450edef68 (cherry picked from commit f2db4112861b10192de7a1a42321b42cb8a2ba55)

[ylavic]

Backported in 4326d6b9041a3bcb9b529f9163d0761c2d760700

[rpluem]

@covener : This one can be closed as it was merged.

[notroj]

Do you not all have power to close these yourselves?

[rpluem]

Unfortunately there was no close button for me. No idea why.