package/proftpd: security bump to version 1.3.6c

package/proftpd: security bump to version 1.3.6c

Fixes the following security issues:

- CVE-2020-9273: In ProFTPD 1.3.7, it is possible to corrupt the memory pool
  by interrupting the data transfer channel.  This triggers a use-after-free
  in alloc_pool in pool.c, and possible remote code execution.

And additionally, fixes a number of other issues.  For details, see the
release notes:

https://github.com/proftpd/proftpd/blob/1.3.6/RELEASE_NOTES

This also bumps the bundled libcap, so
0001-fix-kernel-header-capability-version.patch can be dropped.

While we are at it, adjust the white space in the .hash function to match
the new agreements.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>