Support #14017

    Merge branch '399-merge-dhcpv4-shared-networks-fetched-from-the-cb-into-the-configuration'

[117-configure-script-doesn-t-fail-when-enable-generate-docs-is-used-and-docbook-style-xsl-is-not-installed] Moved the variable after ./configure as recommended by modern autoconfs

[117-configure-script-doesn-t-fail-when-enable-generate-docs-is-used-and-docbook-style-xsl-is-not-installed] Reviewed and completed generate docs description

[117-configure-script-doesn-t-fail-when-enable-generate-docs-is-used-and-docbook-style-xsl-is-not-installed] Made --nonet in xsltproc optional

src/lib/dhcpsrv/cfg_shared_networks.*
    CfgSharedNetworks4::merge() - new method to merge
    shared network configs

src/lib/dhcpsrv/cfg_subnets4.*
    CfgSubnets4::merge() - reworked to reflect new rules

src/lib/dhcpsrv/srv_config.cc
    SrvConfig::merge() - now merges shared networks

src/lib/dhcpsrv/tests/cfg_subnets4_unittest.cc
    checkMergedSubnet() - convenience function for
    verifying subnet content

    TEST(CfgSubnets4Test, mergeSubnets) - revamped to
    reflect new network assigment rules

src/bin/dhcp4/tests/config_backend_unittest.cc
    TEST_F(Dhcp4CBTest, mergeSharedNetworks)  - enabled test

[#372,!181] hammer: reduced line lenghts, added docstrings, unified naming, fixed terminating timed out process