pop3: Fix POP3_TYPE_ANY signed compilation warning

POP3_TYPE_ANY, or ~0, is written to pop3c->preftype in lib/pop3c.c, an
unsigned int variable.  The result of ~0 is -1, which caused a warning
due to writing a negative number to an unsigned variable.  To fix this,
make the expression ~0U so that its value is considered the unsigned
number UINT_MAX which is what SASL_AUTH_ANY does in curl_sasl.h.