Computer-Aided Design, Synthesis, and Anti-HIV-1 Activity in Vitro of 2-Alkylamino-6-[1-(2,6-difluorophenyl)alkyl]-3,4-dihydro-5-alkylpyrimidin-4(3H)- ones as Novel Potent Non-Nucleoside Reverse Transcriptase Inhibitors, Also Active Against the Y181C Variant