(ITS#8982) Fix OpenLDAP headers so they're useful for development

[quanah@openldap.org]

Full_Name: Quanah Gibson-Mount
Version: RE25
OS: N/A
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (47.208.148.239)


It's currently difficult for downstream packagers to create a working
development package as some items required to build external modules are only
included what are considered "private" headers, such as slap.h (See ITS#7283). 
SLAP_CB_{BYPASS,CONTINUE} are examples of items that are contained in slap.h
that are potentially required for building external modules.

IIRC, it's also been discussed previously to rename "ldap_pvt.h" to "openldap.h"
to indicate it's ok to package this header file in a -devel package (I could
have the initial header name incorrect, it's been a while).