Replace text in usage description of fetch that claims to verify sources
with a suggestion to use 'abuild fetch verify', which will actually
verify them.

'abuild fetch' alone will not verify sources, as it only executes the
fetch() function.

This change makes the example in the description of install_if in the
APKBUILD man page match abuild.in:1791.

These are strings after all and should be quoted even if not strictly necessary because of tradition excluding spaces from package names.

Add a '# Check sections' comment, for consistency with the equally
commented build and package sections.

Since the obsolete 'cd "$builddir"' statements have been removed in [1],
build(), check() and package() can generate empty functions if no build
system is specified or if there is no default for the given build
system. newapkbuild will then fail, as it tries to parse the script it
generated:

$ cd /home/pmos && newapkbuild test
/usr/bin/abuild: /home/pmos/test/APKBUILD: line 18: syntax error: unexpected "}"
$ cat test/APKBUILD
...
build() {
}
...

Fix this by placing ":" in functions that would be empty.

[1]: f83d19ce79ab9f2dcc5238346a910cd18ae0f330