rc: Fix calls to mktemp

[lenormf]

Allow mktemp to make use of the TMPDIR environment variable when
calling it with a template.

Don't use the deprecated -t flag.

[casimir]

This PR breaks all the modified scripts on macOS.

Example for lint.kak:

*** This is the debug buffer, where debug info will be written ***
shell stderr: <<<
mktemp: illegal option -- -
usage: mktemp [-d] [-q] [-t prefix] [-u] template ...
       mktemp [-d] [-q] [-u] -t prefix 
mkfifo: /fifo: Permission denied
>>>
Error: 1:1: 'lint' 1:1: 'eval' 1:1: 'write' /buf: Permission denied

[lenormf]

So the mac implementation excepts a -t flag to know what format to use, but the GNU implementation considers this flag deprecated (and will potentially remove it in newer releases) and doesn't seem to take $TMPDIR into account all the time unless the mac-unsupported -p flag is passed…

[mawww]

Can we just use mktemp -d ${TMPDIR:-/tmp}/blah-XXXX in that case ? or will that break on OSX ?

[lenormf]

I think this will work, I'll adapt my changes (although GNU/mktemp expects at least 6 X in the template).

[casimir]

It's working with just -d on OSX.

[lenormf]

It doesn't on all versions, i.e. on OS X 10.10.5.

[mawww]

Whats the problem there ?

[lenormf]

It simply shows a usage message apparently, the template was probably made optional in recent versions.

[lenormf]

Follow up in #1449.