modules/useradd: Allow home dir creation for system users.

If we create user with flag system=True then useradd utility will not
create user home directory even if it defined in login.defs.
This patch adds posibility to create explicitly set home directory (e.g system=/path/to/dir)
for system users.
Merge pull request #3979 from ealekseev/develop

modules/useradd: Allow home dir creation for system users.