fix nipap/sql/Makefile

This should fix some problems in the Makefile.
psql -f does some fancy stuff that does not work on my system:

```
[postgres@localhost sql]$ ls -l
total 84
-rw-r--r-- 1 borgers borgers   936 14. Aug 00:30 clean.plsql
-rw-r--r-- 1 borgers borgers 40474 14. Aug 00:30 functions.plsql
-rw-r--r-- 1 borgers borgers 11348 19. Aug 14:11 ip_net.plsql
-rw-r--r-- 1 borgers borgers  2756 19. Aug 14:33 Makefile
-rw-r--r-- 1 borgers borgers  1621 14. Aug 00:30 upgrade-1-2.plsql
-rw-r--r-- 1 borgers borgers   807 14. Aug 00:30 upgrade-2-3.plsql
-rw-r--r-- 1 borgers borgers  8618 14. Aug 00:30 upgrade-3-4.plsql
-rw-r--r-- 1 borgers borgers   457 19. Aug 14:11 upgrade-4-5.plsql
[postgres@localhost sql]$ make install
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
createuser -S -D -R nipap
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
createdb -O nipap nipap
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
createlang -d nipap plpgsql
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
createlang: language "plpgsql" is already installed in database "nipap"
Makefile:49: recipe for target 'db' failed
make: [db] Error 2 (ignored)
psql -q -c "ALTER USER nipap ENCRYPTED PASSWORD 'XSjSJDflbMXDtAdv'"
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
PGPASSWORD=XSjSJDflbMXDtAdv psql -q -h localhost -U nipap -d nipap -f functions.plsql
could not change directory to "/home/borgers/projects2/NIPAP/nipap/sql": Permission denied
functions.plsql: No such file or directory
Makefile:58: recipe for target 'tables' failed
make: *** [tables] Error 1
```
Would be nice if the makefile works on all systems without much extra work. The piping solution seems to do the job.