newapkbuild: check arguments and improve usage()

Changes:
* argument sanity checks:
  * `PKGNAME[-PKGVER] | SRCURL`
    * check if missing
    * check if specified more than once (see below)
  * specifying more than one buildtype flag
  * `-n` (set pkgname) without using SRCURL as last argument
  * `-s` (sourceforge source) without using PKGNAME as last argument
* Typo fix: exist -> exists
* `usage()`:
  * always print PKGNAME and PKGDESC (instead of NAME and DESC,
    NAME was used in one place and PKGNAME in another)
  * link to <https://spdx.org/licenses/>
  * `-m` (meson) flag was missing in short usage line at the top
  * indicate that the buildtypes are exclusive
  * `-c` flag: remove "to new directory" wording to make the
    message shorter (this should be obvious)
  * remove empty line at the end

NOTE: Before this commit, the `PKGNAME[-PKGVER] | SRCURL` was allowed
to be specified more than once, and the code looped over the arguments.
But this was not documented in `usage()` and had unexpected results:

```
$ newapkbuild first second third
$ tree
.
___ first
    ___ APKBUILD
    ___ first
    ___ ___ APKBUILD
    ___ ___ first
    ___ ___ ___ APKBUILD
    ___ ___ ___ src
    ___ ___ src
    ___ src
```

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