make: use '"filename" line 123' for locations instead of 'filename:123'

The format 'filename:123' is commonly used for identifying a location in
a file. Text editors recognize it and allow quick navigation through it.
The previous format was specific to make and would have required custom
support in editors.

The new format was already used in stack traces, except for the first
line. Now all lines use the same format.

branches: 1.4.2;
tests/make: rename temporary file

The filename extension '.inc' is reserved for files that are checked
into version control.

make: fix filename in warning about duplicate script (since 2018-12-22)

tests/make: refine test for wrong diagnostic line

To trigger the faulty code path, the file where the targets gets its
first command must be included via its relative path. That was the case
when running 'cd usr.bin/make && make test' but not when running the
tests via ATF.

tests/make: demonstrate wrong location in diagnostic (since 2018-12-22)

When a target has multiple places where commands are defined, the
diagnostics mixed up the filename in some cases.