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.

make: on error, print the targets to be made

This helps to understand situations with several nested sub-makes in
varying directories.

branches: 1.6.2;
tests/make: force line-based diagnostics to be listed in the tests

This way, contradictions between the intended output and the actual
output are closer together and have a better chance of being spotted.

make: remove period from end of error messages and warnings

The majority of the existing error messages and warnings does not
include a period at the end. Follow this style consistently.

make(1): error out on .else with extraneous text

make(1): in lint mode, check for ".else <cond>"

make(1): add tests for .else and .for + .endif

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.