Update error message about --datadir
Update error message about --datadir

Noone remembers these days that at some point ZNC supported using the
same config directory, but different znc.conf. So now the old message is
just confusing.

However, nowadays many people confuse "/znc foo" in IRC client with
"znc foo" in shell.

evilnet/znc,badloop/znc,Adam-/znc,Hasimir/znc,psychon/znc,aarondunlap/znc,aarondunlap/znc,KielBNC/znc,jpnurmi/znc,Zarthus/znc,Zarthus/znc,Kriechi/znc,dgw/znc,markusj/znc,Kriechi/znc,markusj/znc,jreese/znc,kashike/znc,Phansa/znc,KiNgMaR/znc,Mikaela/znc,markusj/znc,aarondunlap/znc,Hasimir/znc,kashike/znc,TuffLuck/znc,GLolol/znc,reedloden/znc,KiNgMaR/znc,aarondunlap/znc,Mikaela/znc,Hasimir/znc,badloop/znc,Adam-/znc,KiNgMaR/znc,evilnet/znc,markusj/znc,Mikaela/znc,jpnurmi/znc,BtbN/znc,GLolol/znc,Hasimir/znc,Mikaela/znc,dgw/znc,KielBNC/znc,reedloden/znc,Mkaysi/znc,dgw/znc,Phansa/znc,aarondunlap/znc,Phansa/znc,elyscape/znc,BtbN/znc,reedloden/znc,KiNgMaR/znc,Hasimir/znc,DarthGandalf/znc,reedloden/znc,psychon/znc,Kriechi/znc,elyscape/znc,KiNgMaR/znc,Mkaysi/znc,BtbN/znc,elyscape/znc,elyscape/znc,elyscape/znc,Kriechi/znc,znc/znc,wolfy1339/znc,wolfy1339/znc,Mikaela/znc,jpnurmi/znc,aarondunlap/znc,KielBNC/znc,anthonyryan1/znc,jpnurmi/znc,evilnet/znc,wolfy1339/znc,kashike/znc,DarthGandalf/znc,anthonyryan1/znc,kashike/znc,KielBNC/znc,Phansa/znc,elyscape/znc,KiNgMaR/znc,DarthGandalf/znc,Mkaysi/znc,DarthGandalf/znc,evilnet/znc,psychon/znc,Mkaysi/znc,GLolol/znc,Mkaysi/znc,Kriechi/znc,jreese/znc,Phansa/znc,Zarthus/znc,Kriechi/znc,psychon/znc,anthonyryan1/znc,BtbN/znc,znc/znc,Mkaysi/znc,TuffLuck/znc,KielBNC/znc,reedloden/znc,DarthGandalf/znc,dgw/znc,znc/znc,Phansa/znc,wolfy1339/znc,TuffLuck/znc,wolfy1339/znc,wolfy1339/znc,Adam-/znc,GLolol/znc,Mkaysi/znc,BtbN/znc,Hasimir/znc,jpnurmi/znc,evilnet/znc,evilnet/znc,jreese/znc,Hasimir/znc,dgw/znc,Mikaela/znc,psychon/znc,GLolol/znc,kashike/znc,badloop/znc,reedloden/znc,Adam-/znc,badloop/znc,BtbN/znc,badloop/znc,GLolol/znc,Adam-/znc,TuffLuck/znc,anthonyryan1/znc,DarthGandalf/znc,badloop/znc,jreese/znc,znc/znc,Zarthus/znc,markusj/znc,anthonyryan1/znc,Adam-/znc,TuffLuck/znc,kashike/znc,TuffLuck/znc,jreese/znc,KielBNC/znc,markusj/znc,Adam-/znc,Zarthus/znc,Zarthus/znc,anthonyryan1/znc,Mikaela/znc,aarondunlap/znc,jpnurmi/znc,znc/znc,jreese/znc,anthonyryan1/znc,dgw/znc,TuffLuck/znc,znc/znc,badloop/znc,psychon/znc,jpnurmi/znc