hostname: Remove redundant codepath and replace with FormatMessage

[Hugh McMaster]

Removes the redundant vsprintfW codepath and replaces it with FormatMessage.

Precedent for these changes:
attrib:      6aacb090f6bad8f3109cea710cc3aab462eb97d4
ipconfig:  66b06a499d97338dc5ef0b5c4c78b107d577ced8
net:         f3f24fb5d4b017e026333c5d00066d727741a4b5
xcopy:     0cdabdd64873439052b3efa84850bc08cacf5395
 		 	   		  
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