Doing this with POSIX is tricky:
POSIX Sed does not support \r or \15. Even if it did, the in place
option -i is not POSIX
POSIX Awk does support \r and \15, however the -i inplace option
is not POSIX
d2u and dos2unix are not POSIX utilities, but ex is
POSIX ex does not support \r, \15, \n or \12
To remove carriage returns:
awk 'BEGIN{RS="^$";ORS="";getline;gsub("\r","");print>ARGV[1]}' file
To add carriage returns:
awk 'BEGIN{RS="^$";ORS="";getline;gsub("\n","\r&");print>ARGV[1]}' file