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

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).