build/meson: cleanup "meson-post-install.sh"

- the variables in meson.build and in the meson-post-install.sh script
  should have the same names.

- the positional command line arguments should be assigned to variables,
  because the variable name acts like a documentation what the variable
  means (contrary to the argument number).

- the boolean flags should not map to other special values, like
  "enable_docs ? 'install_docs' : ''". The name "enable_docs" is
  good already, it shall be either passed as 1 or 0 and use the name
  consistently.
18 jobs for th/connectivity-cleanup in 33 minutes and 35 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Test
passed #1003994
allowed to fail
checkpatch

00:01:22

manual #1004000
allowed to fail manual
t_centos:7.5.1804
manual #1004001
allowed to fail manual
t_centos:7.6.1810
manual #1004007
allowed to fail manual
t_debian:10
manual #1004006
allowed to fail manual
t_debian:9
manual #1004009
allowed to fail manual
t_debian:sid
manual #1004008
allowed to fail manual
t_debian:testing
manual #1003995
allowed to fail manual
t_fedora:28
manual #1003996
allowed to fail manual
t_fedora:29
passed #1003997
t_fedora:30

00:33:07

manual #1003998
allowed to fail manual
t_fedora:31
manual #1003999
allowed to fail manual
t_fedora:rawhide
manual #1004002
allowed to fail manual
t_ubuntu:16.04
manual #1004003
allowed to fail manual
t_ubuntu:18.04
manual #1004005
allowed to fail manual
t_ubuntu:devel
manual #1004004
allowed to fail manual
t_ubuntu:rolling
 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/910/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/910/

08:15:27