The President may prescribe regulations necessary to carry out this subchapter and section 3343 of this title and to protect and assure the retirement, insurance, leave, and reemployment rights and such other similar civil service employment rights as he finds appropriate. The regulations may provide for the exclusion of employees from the application of this subchapter and section 3343 of this title on the basis of the nature and type of employment including excepted appointments of a confidential or policy-determining character, or conditions pertaining to the employment including short-term appointments, seasonal or intermittent employment, and part-time employment.